1. Field of the Invention
The present invention relates to a process for preparing a phosphorus-containing compound, particularly to a process for preparing 10-chloro-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide compound.
2. Description of the Prior Arts
As public environmental awareness increases, many halogen-containing flame retardants have been gradually replaced by halogen-free compounds to avoid producing highly toxic and corrosive gases during combustion of polymers that contain halogen type flame retardants. The halogen-free compounds are commonly used to improve the properties of the obtained polymers such as flame retardancy and heat resistance, and the halogen-free compounds also render the obtained polymers more environmentally friendly.
One of the most widely studied halogen-free compounds is 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), a phosphorus-containing compound which can be used as a phosphorus based flame retardant. However, DOPO and its derivatives are well suited only for limited polymers. In order to further expand the application of DOPO that can be successfully applied in various kinds of polymers, researchers have been focused on developing various derivatives of DOPO.
One of the derivatives of DOPO is 10-chloro-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (abbreviated as DOPO-C or CDOPO), which also itself can be used as a phosphorus based flame retardant and is a reactive flame retardant intermediate as well. By reacting of DOPO-C with polymers, a halogen-free flame retardant system can be thus produced. For example, a phosphorus-containing phenolic resin can be obtained by reacting DOPO-C with phenol formaldehyde resin, which can be used as a non-halogenated curing agent for copper clad laminate and printed circuit board industries.
Accordingly, due to the promising and potential applications of DOPO-C for electrical and electronic industries, researchers from academia and industry have been improving and developing new synthetic processes of DOPO-C. As disclosed in U.S. Pat. No. 7,232,921, a method for preparing a biphenylphosphonate compound involves oxidizing 6-chloro-6H-dibenz[c,e][1,2]oxaphosphorin (CDOP) by using ozone under heat, so as to obtain DOPO-C. However, the drawbacks of the method disclosed by the prior art include: complicated precision instruments are required for preparing ozone and adjusting ozone in amount, thereby remarkably increasing the difficulty of preparation; and excessive exposure to hazardous ozone gas will harm the respiratory system, mucous membrane and eyes. Therefore, because of the costly precision instruments, complexity in operating the precision instruments, and chemical hazard, the conventional method is not applicable for mass production of DOPO-C.
Besides, Jörg Gloede et. al. (Z. Anorg. Allg. Chem. 2003, 629, 998-1000), disclosed a method for preparing DOPO-C. CDOP is reacted with chlorine in chloroform at a low temperature of −20° C. at first, and DOPO-C is obtained after hydrolysis reaction. However, the drawbacks of the method disclosed by the prior art include: a low yield of only 76%; the difficulty in operation of gaseous chlorine; and irritation of the respiratory system by chlorine, all of which result in harming health and safety of workers, and may further lead to death; and a requirement of precision instruments such as low temperature thermostatic sinks since a low temperature condition is needed in the course of preparing DOPO-C. Therefore, the aforementioned disadvantages lower the industrial application of the method and render the method unsuitable for mass production of DOPO-C as well.
To overcome the shortcomings, the present invention provides a process for preparing 10-chloro-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO-C) compound to mitigate or obviate the aforementioned problems.